For the large-scale integrated circuit (LSI), the phenomenon in which the memory content disappears temporarily without accompanying physical damage of the device structure is known as a soft error. The main reason for a soft error is an alpha ray which is generated by the minute amount of radioactive nuclear seeds contained in the material of the LSI.
Among the materials that form the LSI--for example, silicon, oxygen, nitrogen, boron, phosphorus, arsenic, aluminum, titanium, tungsten, and copper--there are no natural radioactive isotopes, and the main sources of the alpha ray are uranium (U) and thorium (Th) contained as impurities. In order to alleviate the occurrence of a soft error in the LSI, it is necessary to reduce the content of uranium and thorium to the ppb level in the structural materials of the LSI.
On the other hand, there is still a high demand for increasing the degree of integration of the LSI. For this purpose, the oxide film-nitride film-oxide film structure (the so-called ONO structure) is adopted, and the steric structure of the cell capacitor of the dynamic random-access memory (DRAM) becomes more complicated. The reason is as follows: as the effective specific dielectric constant is selected at about 5 for the ONO structure, in order to ensure the electrical capacitance of the cell capacitor with respect to the shrinking cell area, efforts can be made only on the shape of the cell capacitor in order to guarantee the area. This, however, increases the burden on the manufacturing operation of a DRAM with respect to both technology and cost.
In order to solve this problem, oxides with a high permittivity such as tantalum oxide (Ta.sub.2 O.sub.5), strontium titanate (SrTiO.sub.3), BST (Ba.sub.x Sr.sub.(1-x) TiO.sub.3), PZT (PbZr.sub.x Ti.sub.(1-x) O.sub.3), etc., have been considered as insulating materials of a memory cell capacitor.
The most significant difference between these substances and silicon oxide and nitride is that these substances have a higher oxidizing power for silicon. Consequently, using a silicon electrode with these new types of insulating materials and selecting a type of material with a high oxidation resistance as the electrode is possible. At present, platinum is believed to be the candidate for this purpose.
However, platinum differs from the conventional elements used as the materials for LSIs in that it contains a radioactive isotope that emits alpha rays, that is, Pt.sup.190 that can emit alpha rays with a kinetic energy of 3.18 MeV in 5.4.times.10.sup.11 of radioactive half-life. The content of Pt.sup.190 in platinum is 0.013%, and it is very difficult to isolate it when cost is a consideration. If the alpha ray level cannot be alleviated using certain methods, it is impossible to reduce the soft error level to the appropriate level for commercially using the platinum electrodes in manufacturing DRAMs.
For example, when a 2000-.ANG.-thick platinum film is formed on a silicon substrate, the dose of the alpha ray emitted from the platinum film into silicon is 0.02 photon/h.multidot.cm.sup.2. That is, one alpha ray photon on 10 cm.sup.2 for 50 h. This calculated result is the number of alpha ray photons emitted from Pt into silicon.
For the DRAM, the soft error should be in the range of 500-1000 FIT (failure in time, where 1 FIT means that a defect takes place in 1 device among 10.sup.9 devices in 1 h). That is, 1000 FIT means that one soft error takes place in 1000 h for 1000 DRAMs. When the size of the DRAM's cell array portion is about 1 cm.sup.2, the frequency for the soft error is on the order of about 10.sup.-6 photon/h.multidot.cm.sup.2. Although not every alpha ray photon can lead to a soft error, the aforementioned value of 0.02 photon/h.multidot.cm.sup.2 (the measured value is 0.007 photon/h.multidot.cm.sup.2) is still too high. On the other hand, if the amount of U, Th, and other radioactive elements in platinum can be kept to less than 10 ppb, it is acceptable to ignore the influence of the alpha ray caused by these radioactive elements.
Let us discuss the effect in alleviating the influence of the alpha ray generated by the platinum electrode in the conventional structure. When a ferroelectric material and platinum electrode are used corresponding to the DRAM, the COB (capacitor over bitline) structure shown in FIG. 16 is adopted for the following two purposes: to ensure the area of the cell capacitor and to prevent degradation of the cell capacitor portion in the high-temperature heat treatment for forming the other portions.
In this COB structure, a stack-type cell capacitor CAP is formed on the upper portion of wordline WL and bitline BL. Polysilicon 20 is used as the contact between the lower (platinum) electrode 16 of cell capacitor CAP and n.sup.+ -type diffusion layer 3 (source region) of silicon substrate 1. In FIG. 16, 2 represents a field oxide film; 4 represents an n.sup.+ -type diffusion layer (drain region); 5 represents a gate oxide film; 8 represents an upper electrode; 10 and 10' represent interlayer insulating films; 11 and 12 represent contact holes; 17 represents a dielectric material (such as PZT) film; and TR represents a transfer gate.
However, for the aforementioned cell capacitor CAP, the height of polysilicon contact 20 arranged beneath lower (platinum) electrode 16 is at most 1 .mu.m. By means of this silicon portion, the energy of the alpha ray generated by Pt.sup.190 in lower electrode 16 can be reduced to only about 3 MeV. This energy is still high enough to cause a soft error.
It is an object of this invention to provide a type of capacitor, electrode or wiring structure, and a DRAM or other semiconductor device using such structures in which the alpha ray is effectively shielded in order to prevent the occurrence of a soft error even when platinum or some other electroconductive material is used as the electrode of the capacitor or in the wiring structure.